1. Field of the Invention
The present invention is directed to a method, in the form of a pulse sequence, for acquiring raw data for use in magnetic resonance imaging. The present invention is also directed to a magnetic resonance tomography apparatus operating according to the inventive method.
2. Description of the Prior Art
An especially fast method for acquiring image information in magnetic resonance apparatus is known as the turbo-spin echo method and is disclosed, for example, in U.S. Pat. No. 5,545,990. In this method, nuclear spins are excited with a 90xc2x0 radio-frequency excitation pulse and are subsequently refocused by a number of 180xc2x0 radio-frequency refocusing pulses. This is repeated at successive times defined by the repetition time until all data for a complete magnetic resonance image have been acquired. This method is significantly faster than conventional spin echo sequences wherein only one refocusing ensues after every excitation, i.e. only one nuclear magnetic resonance signal is acquired. The turbo-spin echo method, however, is usually employed for T1 and proton density contrast and exhibits a non-negligible loss of resolution compared to the spin echo method.
It is an object of the present invention to provide a method, in the form of a pulse sequence, for acquiring raw data for magnetic resonance imaging wherein the above-described loss of resolution in a turbo-spin echo sequence is avoided.
It is a further object of the present invention to provide a magnetic resonance imaging apparatus, operable using a pulse sequence based on the known turbo-spin echo sequence, wherein the above-discussed loss of resolution associated with the traditional turbo-spin echo sequence is avoided.
The above objects are achieved in accordance with the invention in a method and apparatus wherein an examination subject is irradiated with a number of RF excitation pulses with a repetition time between the pulses and with an RF refocusing pulse emitted after each RF excitation pulse, with at least one magnetic resonance echo signal being acquired after every RF refocusing pulse, with each magnetic resonance echo signal being phase-encoded in at least one direction by a predetermined phase-encoding gradient, and wherein the nuclear magnetic resonance signals are sampled, digitized and phase-sensitively demodulated and are entered into rows of a raw data matrix in an order corresponding to their phase-encoding, and wherein the phase-encoding of the magnetic resonance echo signals is implemented in a time sequence so that the echos which occur after each excitation are allocated to segments of interconnected (successive) rows of the raw data matrix.
Differing from the traditional turbo-spin echo method, the acquired nuclear magnetic resonance signals in the inventive method and apparatus are not entered into segments of the raw data matrix according to the echo number within the individual excitations, but instead are grouped according to the excitations. The edge rows of the raw data matrix thus exhibit an amplitude that is comparable to the central rows of the raw data matrix. Since the edge rows determine the resolution of the image, the resolution is correspondingly improved.
The number of radio-frequency refocusing pulses emitted after each radio-frequency excitation pulse can be increased for radio-frequency excitation pulses which occur later in the overall data acquisition scan. Since the central rows of the raw data matrix determine the contrast, echo trains that are arbitrarily long cannot be employed for these central rows without falsifying the T1 or proton density contrast. The later nuclear magnetic resonance signals allocated to the edge rows of the raw data matrix, however, have a determining effect which is critical only for the resolution, so that longer echo trains, i.e. a greater number of radio-frequency refocusing pulses, can be employed for registering these.
Amplitude discontinuities in the raw data matric can be avoided by an averaging of measured values in an embodiment of the invention.